Although most of the oxygen consumed by the brain is used in the production of energy, a small portion of the oxygen is utilized as a substrate in the synthesis of the neurotransmitters, norepinephrine, dopamine and serotonin. The rate-limiting steps in the production of these monoamines are mixed-function oxygenases, tyrosine and tryptophan hydroxylase. Previous work in this laboratory and by others both in vivo and in vitro, has suggested that these enzymes are not saturated with oxygen in animals breathing room air. The hypothesis underlying this proposal is that nerve stimulation effects an allosteric activation of these hydroxylases increasing their affinity for oxygen and allowing an increase in neurotransmitter synthesis despite limiting concentrations of tissue oxygen. The object of this study is to describe the activation of tryptophan hydroxylation by hyperthermia, a condition known to increase the firing of serotonergic neurons in the raphe nucleus of the rat. I wish to determine the kinetic characteristics of tryptophan hydroxylase for oxygen and tryptophan in vivo in normothermic and hyperthermic rats. I will compare the characteristics of "activated" enzyme in vivo to kinetic measurements of tryptophan hydroxylase enzyme in vitro from hyperthermic rats and to similar measurements of enzyme from normothermic rats in the presence of various factors, such as cyclic nucleotides and ions. This correlation of in vivo and in vitro data may allow a greater understanding of the biochemical mechanism underlying the activation of these mixed-function oxygenases by neuronal discharge. These experiments will provide information basic to defining the brain's response to oxygen deprivation.